Frequency doublers are widely used to extend the upper frequency of fixed or variable oscillators. Typically, frequency doubling is realized by feeding the signal through nonlinear devices, such as diodes or transistors, and then extracting the second harmonic and rejecting other frequency components.
The basic problem in designing a doubler is that the fundamental frequency energy appears at the output port. This is especially a problem when the bandwidth approaches an octave so that the highest frequency at the input band overlaps that of the lower band of the output. Typical of previous solutions employed are those disclosed in papers by C Rauscher entitled "High Frequency Doubler Operation of GaAs Field-Effect Transistors", IEEE Trans. Microwave Technology and Techniques, volume 31, No. 6, June 1983, pp. 462-473 and another by P. D. Chow, et al., "A 44 GHz HEMT Doubler/Amplifier Chain" IEEE Int. Symp on Microwave Technology and Techniques, 1990, pp 603-606.
In such previous designs, circuit components such as micro strip quarter-wave length or radial open stubs are used in the output port of the doublers, which shunt the fundamental frequency component to ground to prevent it from appearing at the output. Similar techniques may also be used at the input port to short-circuit the second harmonic to ground to achieve good input-output port isolation.
However, these techniques are limited to narrow band applications, because micro strip quarter-wave length open stubs are difficult to realize over a wide range configuration. The state of the art of such circuits are described in a publication by R. Gilmore, entitled, "Concepts in the Design of Frequency Multipliers", in the Microwave Journal, March 1987, pp 129-139.
In these circuits, baluns (or transformers) are employed to couple input signals to the non-linear devices, and to couple the energy at the wanted second harmonic frequency to the output ports. Since the configuration inherently suppresses the fundamental and odd harmonics, wider bandwidth can be achieved. Unfortunately, there are some drawbacks associated with these circuits. Firstly, the loss of the baluns used at the input and/or output ports inevitably degrades the conversion efficiency of the doubler. Secondly, the use of the baluns increases considerably the complexity and physical dimensions of the doubler circuit.
It is an object of the present invention to provide an improved microwave frequency doubler circuit.